Here’s a really useful case study for anyone who wants to do “guerrilla” responsive design: when you’re handed a fixed-width mockup but you know that responsive is the way to go:

I started by styling up every element, without layout. The result was a fully elastic layout that effectively served as a mobile, or small screen, layout, which just needed some tweaking of horizontal spacing.

Bingo! And this approach had knock-on benefits as it “supported writing component-based, or modular, CSS”.